Tear-off lids are known, for example, from DE-U 298 17 592 or from DE-U 92 03 953. A production of tear-off lids according to the prior art and known to a person skilled in the art will now be briefly described making reference to FIGS. 1 to 6. FIG. 1 shows a lid blank 1. Shown at the beginning of the transport route of a known production apparatus 20 is a stack 12 which contains a plurality of lid blanks of this type, said blanks being removed individually and processed by various processing stations along the transport route wherein initially lid rings are formed from the lid blanks and the lid rings are then further processed into the finished tear-off lid. The lid blanks are, for example, round disks made of coated metal which is thereby protected against corrosion, in particular coated tinplate. They have, for example, a diameter of 11 cm. These blanks have already been pre-formed at the edge 6 thereof by a processing machine (not shown) and the shaping of the edge 6 later serves for fastening the finished tear-off lid to a container or a can by means of a seam joint. This is known to a person skilled in the art and will not be described further here.
The conveying device 22 which conveys the lid blanks, the lid rings and the tear-off lids in the production apparatus 20 along the transport route in the direction of the arrow A from one processing station to the next processing station is provided in particular by two toothed belts which run in parallel and on which receptacles for the lid blanks or lid rings are provided, as is known to a person skilled in the art from WO 2006/017953. This also will not be described further here. At each of the processing stations according to the prior art, which are known to a person skilled in the art and are shown purely schematically here, each blank to be processed is lifted off the conveying device and is processed by the processing station and then returned to the conveying device. This is indicated at the processing stations with arrows pointing up and down. At the processing stations, the drive of the processing stations is indicated in each case under the conveying means. This serves to raise and lower the blanks and the lid rings and to carry out the respective processing steps.
In a stamping processing station 23, a lid ring 2 is formed from the lid blank 1 in that a central part 30 of the blank is cut out and disposed of as waste. In this way, the removal opening of the tear-off lid, which in a later production step will be closed with the tear-off film, is formed. A sealing flange 7 remains adjacent to the removal opening 15. At the cut edge 31, the metal material is bare and the sheet metal of the lid ring 2 is no longer protected there by the coating. In a processing station 24, the edge of the removal opening is pulled up to form a collar 4 and this collar is curled in a further processing station 25 so that a so-called “retort curl” 5 is formed. The form of the curl, or retort curl, can vary. The curl which forms the edge of the removal opening ensures that the user of the can is protected from the sharp cut edge when removing the contents of the can.
The production steps described so far were carried out in a position in which the lid blanks and lid rings were arranged with their later upper side or with the sealing flange 7, respectively, facing downwardly. This is also the preferred configuration in the present invention. As mentioned, the blank or the ring is lifted into the individual processing stations 23, 24 and 25 from the conveying device, processed and returned again, whereupon the conveying device carries out conveying steps which guide the lid blank or lid ring to the next processing station. If the production steps are carried out, as shown, with the sealing flange 7 facing downwardly, then a turning station 26 follows, which turns the lid rings such that during further processing, following the turning station, the sealing flange 7 lies in a position to face upwards in the conveying device 22 and in the processing stations.
Thereafter, the tear-off film 8 is sealed onto the sealing flange, and this can be carried out in two steps with a pre-sealing station 27 and a main sealing station 28. The sealing process is also known to a person skilled in the art and will not be described further here. Further processing stations can follow in which an embossing of the sealing foil can take place, the tear-off flap is positioned and a seal tightness test is carried out. This is also known to a person skilled in the art and will not be described further here. At the end of the known production apparatus 20, finished tear-off lids 10 are output, their removal opening 15 being spanned by a tear-off film 8 which is sealed onto the sealing flange 7. The edge of the removal opening 15 is formed by an upwardly and outwardly bent-over retort curl 5. The finished tear-off lid 10 can be fastened, by means of the fold edge 6 thereof, to a can body (which is indicated in FIG. 5 with a body wall part 11 only) and thereby closes the can. This is achieved during the filling operation after the can has been filled with a filling product. The can, once filled and closed, can be opened later in that the tear-off film is torn off from the lid ring by means of the tear-off tab 18, so that the removal opening 15 is opened. Tear-off lids of this type have proved to be effective.
If the container which is closed with the tear-off lid contains a liquid, corrosive filling product, for example, salt water, the bare cut edge 31 can corrode. Although, due to the upward and outward curl and with the covering by the tear-off film in the case of the tear-off lid as described with the retort curl, a certain degree of protection is provided against the influence of the filling product and the bare and possibly corroded cut edge is not visible as well, but depending on the aggressiveness of the filling product and on the storage time of the filled container, traces of the corrosion can nevertheless become visible. In EP-A 1 153 840, it is proposed that the tear-off film is also sealed onto the curl itself so that the cut edge is protected against the can contents by sealing. However, it has been found that sealing both on the sealing flange and also on the curl is barely achievable with reliability in production at a high production rate. WO-A 02/790041 mentions incidentally that the gap arising due to the curl can become filled with the hot seal coating of the tear-off film, but without explaining how this could actually be achieved during the industrial production of tear-off films. Known tear-off films are provided with only a very thin hot seal layer and it is not evident to a person skilled in the art how this layer could be used for gap filling when a hot sealing process known to the skilled person is used. The solution suggested in WO-A 02/790041 appears therefore to be unusable in practice. From EP-B 2 055 736, it is known to provide a container with an outward curl at the mouth of the container, so that the cut edge does not come into contact with water. It is proposed to provide protection of the cut edge there with a modified hot melt material which contains a thermoplastic elastomer.